What is operating system? Describe its functions and classifications.

An Operating System (OS) is system software that manages hardware and software resources and provides an interface between user and computer. Examples: Windows, Linux, Android.

Functions:

1. Process management — Schedules CPU among tasks; handles multitasking and termination.
2. Memory management — Allocates RAM, tracks free/used memory, uses virtual memory, protects programs from each other.
3. File management — Creates, stores, searches and secures files with permissions (e.g. NTFS).
4. Device management — Controls I/O via drivers with spooling/buffering (e.g. print queue).
5. Security — User accounts, passwords, access rights; protects from unauthorized use.

Classification:

1. By users/tasks: Single-user single-task (MS-DOS), single-user multi-task (Windows 11), multi-user (Linux server, Unix) — many users share one powerful machine.
2. By interaction: GUI (Windows, icons/mouse) vs CLI/CUI (DOS, Linux terminal commands).
3. By purpose: Real-time OS (RTOS — immediate response for aircraft, medical devices), Network OS (manages network resources), Mobile OS (Android, iOS), Distributed OS (many machines act as one).

Exam close: offices use multi-tasking GUI OS (Windows/Linux); servers use multi-user OS; devices use RTOS.